Natalie Wood's death certificate has been changed to reflect some of the uncertainties and lingering questions surrounding the actor's drowning more than 30 years ago in the Pacific Ocean off southern California.

The document was amended this month and shifts the cause of Wood's death from accidental drowning to "drowning and other undetermined factors", according to a copy of the certificate.

The amended document states that the circumstances of how Wood ended up in the water off Catalina island in November 1981 are "not clearly established".

The changes occurred nine months after investigators renewed their inquiry into Wood's death shortly before its 30th anniversary.

The Los Angeles county sheriff's chief of detectives, William McSweeney, said the decision to amend the death certificate was ultimately made by the coroner's office, which has been instructed by detectives not to discuss the case. "I would just say undetermined is descriptive," he said.

McSweeney said detectives still had work to do on the case but that doesn't necessarily mean that a major shift is coming.

"We don't close these cases," he said. "These cases have active periods and more passive periods. We're moving toward the end of an active period."

The changes to the death certificate were approved by the Los Angeles county chief medical examiner, Dr Lakshmanan Sathyavagiswaran, in late June, but they were not formally recorded until 1 August.

Conflicting versions of what happened on the yacht shared by Wood and two other actors – her husband Robert Wagner and their friend Christopher Walken – have contributed to the mystery of how Wood died on Thanksgiving weekend in 1981. Authorities have said Wagner is not a suspect in his wife's death.

Investigators reopened the case last November but have released few details about its progress. They travelled to Hawaii to inspect the Splendour, the yacht the trio were aboard on the night of Wood's disappearance.

The renewed inquiry came after the boat's captain, Dennis Davern, told 48 Hours Mystery and the Today show he had heard Wagner and Wood arguing on the night of her disappearance and believed Wagner was to blame for her death.

Wagner wrote in a 2008 memoir that he and Walken had argued that night. He wrote that Walken had gone to bed and he had stayed up for a while, but when he went to bed he noticed that his wife and a dinghy attached to the yacht were missing.

Wagner has said through a spokesman that he expected the sheriff's department to conduct a fair investigation.

Wood was nominated for three Academy Awards during her lifetime. Her death at the age of 43 stunned the world and has remains one of Hollywood's most enduring mysteries. The original detective on the case, Wagner, Walken and until recently the coroner's office have all said they considered her death an accident.